implement sparse_vector example with merge_by_key

[GoogleCodeExporter]

Currently uses sort_by_key, which doesn't exploit the sorted nature of the 
input sequences.

Original issue reported on code.google.com by wnbell on 18 Feb 2012 at 8:10

• Blocked on: #393

[GoogleCodeExporter]

Forwarded to https://github.com/thrust/thrust/issues/64

Original comment by jaredhoberock on 7 May 2012 at 9:05

• Changed state: Forwarded